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Wideband bandstop frequency selective structures

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Author

Ali Al-Sheikh

Date of Issue

2015

School

School of Electrical and Electronic Engineering

Abstract

Frequency selective surfaces brought several design challenges to light, and have been central to much research recently. Known limitations are: poor filtering characteristics; sensitivity to large oblique incident angles when used as both filters and polarization manipulators, which results in a shift frequency response and deteriorated in- and out-of-band transmission and reflection responses; and the inability to maintain a satisfactory angular performance over a very wide bandwidth. This thesis introduces three novel designs based on a new class of three-dimensional frequency selective structure to present promising wideband bandstop structures suitable for different applications, following simple design procedures.
To fulfill the wideband requirements in many applications, and since bandstop structures with very wide band and stable response have not been reported yet, ultra-wide bandstop structures with good angular stability are proposed in this thesis. Providing a wide band and an angular stable response is a challenging task as the response of the upper part of the band deteriorates due to relatively larger unit-cell sizes. Therefore, using a novel and common design procedure based on a parallel strip line (PSL) unit-cell, two structures are proposed using different concepts, exhibiting ultra wide bandstop responses and different unit-cell thicknesses and angular stability levels. The main advantage of the PSL is that the unit-cell size perceived by the incident E-field has no effect on the frequency response and bandwidth, giving us freedom to control its size for an improved angular stability. The first design exploits higher order harmonics to achieve a fractional bandwidth of 78\%. It features a new design approach for this class of structures to excite and properly position the harmonic to widen the bandwidth. It also provides a wide harmonic-free out-of-band response and represents a good solution when better out-of-band responses are in demand. The second design involves a cascaded structure of stacked PSL unit-cells. The fractional bandwidth attained is 100\% with good angular stability up to 60$ ^{o} $ over the entire band. Both structures are relatively superior to the state of the art alternatives in terms of bandwidth, out-of-band performance, and angular stability.
Polarization rotation function has also been introduced to the bandstop structure to demonstrate a stable rotator with good cross-polarization isolation utilizing a parallel plate waveguide (PPW). It is based on the same design procedure proposed for the first two structures. An L-shaped slot is etched on one of the PPW's sides to trap and rotate the orthogonal incident electric field component. The realized rotator has an insertion loss of 1.3 dB at 10 GHz and exhibits a good angular performance that outperforms the available alternatives in angular stability. Due to the structure's stop band performance, it suppresses the co-polarized field component at the frequencies surrounding the center frequency of the rotated component, improving the cross-polarization isolation.
Finally, a few topics are suggested regarding related future works. They involve other ultra-wide bandstop solutions with improved bandwidth, as well as conformal and multi-band variations of frequency selective structures.